Printed circuit board and manufacturing method thereof

ABSTRACT

A printed circuit board includes a flexible insulated substrate with a first surface and a second surface at both sides respectively, a wiring layer on the first surface, a reinforcement plate on a part of the second surface and an auxiliary layer between the second surface and the reinforcement plate. A reinforcement edge side of the reinforcement plate is located at the outside of an auxiliary edge side of the auxiliary layer.

This application is based upon Japanese Patent Application No.P2005-242494, filed on Aug. 24, 2005, in the Japanese Patent Office, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printed circuit board, morespecifically to a flexible printed circuit board including areinforcement board and a manufacturing method thereof.

2. Description of the Related Art

Recently, because of miniaturization of elements and reduction in thesize of electronic devices, such as portable telephones and digitalcameras, the interior space in these electronic devices has decreased.Therefore, it is difficult to provide electronic elements, such as aprinted circuit board or a wiring board in such a small space.

Generally, a flexible board, such as a FPC (a flexible printed circuitboard) and an FPWB (a flexible printed wiring board) is used widely as asuitable solution for such a narrow, small space. The FPC may bemodified with a reinforcement board attached at, for example, aconnector part or an element mounting part of the board in order toincrease mechanical integrity and to increase the rigidity in theportions.

FIGS. 1A and 1B (prior art 1) and 2A-2C (prior art 2) illustrate thetechnology of a FPC with the reinforcement board in prior art.

In the FPC shown in FIG. 1A, a wiring layer 22 is formed by patterning acopper foil adhered to a insulated substrate 21 made of a polyimideresin. The sections of reinforcement board 23 a and 23 b are formedcorrespond to the reinforcement portions X1 and X2 in the wiring layer22, as a connector part which needs to be reinforced. The reinforcementboards 23 a and 23 b are arranged to face the adhesive layers 24 a and24 b provided on a face of each reinforcement board 23 a and 23 b in thedirection of an undersurface of the insulated substrate 21.

Positioning holes 25 a 1 and 25 b 1 in the insulated substrate 21 andthe positioning holes 25 a 2 and 25 b 2 in the reinforcement boards 23 aand 23 b overlap with each other to receive the guide pins 26 a and 26b, respectively. Then, the reinforcement boards 23 a and 23 b arepositioned to face the reinforcement portions X1 and X2. As shown inFIG. 1B, the reinforcement boards 23 a and 23 b are pressed towards theundersurface of the insulated substrate 21 under high temperature andhigh-pressure conditions, and secured thereto by the adhesives layers 24a and 24 b, In this case, the guide pins 26 a and 26 b are removed. Thetechnique to attach the sections of reinforcement board to the insulatedsubstrates by the adhesive layers is described in Japanese PatentLaid-Open Publication No. 2002-314207.

A technique developed by the present inventor is shown in FIG. 2A. Areinforcement substrate 33 is adhered by thermo-compression bonding, forexample, to an undersurface of a polyimide resin insulated substrate 31that has a copper foil 32 a on the upper surface. A wiring layer 32 isformed by patterning the copper foil 32 a as shown in FIG. 2B. As shownin FIG. 2C, the photo-mask layers 34 and 35 are formed on anundersurface of the reinforcement substrate 33, corresponding to thereinforcement portions X1 and X2, so as to increase structuralintegrity. The reinforcement plates 33 a and 33 b are formed by chemicaletching on the exposed portion of the reinforcement substrate 33.

The insulated protection layers 27 and 36 on the wiring layers 22 and32, such as a solder resist, are shown in FIG. 1A and FIG. 2C,respectively, and the deformable portion is indicated as Y.

However, according to the prior art 1 shown in FIG. 1A, the adhesiveslayers 24 a and 24 b spread along the undersurface of the insulatedsubstrate 21 through the side of the reinforcement plates 23 a and 23 b,due to applied pressure at the time of adhesion. The overflow portions29 a and 29 b are generated due to the applied pressure, which mayadhere unnecessary materials that prevent the deformation of theinsulated substrate 21.

Moreover, positioning of the reinforcement plates 23 a and 23 b requiresthe positioning holes and the guide pins so that the manufacturingoperation is complicated. There is a problem that positioning accuracyis as low as about 100 micrometers positioning gap. As the printedcircuit board becomes smaller, the problem becomes more serious. Whenthe positioning is performed by visual adjustment, the accuracy isfurther decreased.

As shown in FIG. 2C, the positioning accuracy is improved in the priorart 2 since it is based on photolithography technology. But, since theetching near the insulated substrate 31 is slow, a taper etching of theside wall of the reinforcement plates 33 a and 33 b is carried out andthe skirt portions 36 a and 36 b of each taper, which approaches andextends toward the insulated substrate 31 may form a sharp edge.Therefore, when the insulated substrate 31 is twisted and bent on anapparatus, the insulated substrate 31 may be damaged and develop afrature in response to the concentration of stress at one point.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a printed circuitboard comprises a flexible insulated substrate with a first surface anda second surface at both sides, respectively, a wiring layer on thefirst surface, a reinforcement plate on a part of the second surface andan auxiliary layer between the second surface and the reinforcementplate. A reinforcement edge side of the reinforcement plate is locatedat the outside of an auxiliary edge side of the auxiliary layer.

According to a second aspect of the present invention, a manufacturingmethod is provided for a printed circuit board with a flexible insulatedsubstrate having a first surface and a second surface at the sidesthereof respectively. The method comprises forming a wiring layer bypatterning on the first surface of the flexible insulated substrate,placing a reinforcement material on the second surface of the flexibleinsulated substrate via an auxiliary material, forming a pattern masklayer to partially expose a surface of the reinforcement material andshaping an auxiliary layer and a reinforcement plate from remainingparts of the materials by etching the exposed surface. The etchingprocess for the auxiliary material is faster than the reinforcementmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a process sectional view showing a process of themanufacturing method of the printed circuit board according to the priorart.

FIG. 1B is a process sectional view showing a process of themanufacturing method of the printed circuit board according to the priorart.

FIGS. 2A-2C are process sectional views showing a process of themanufacturing method of the printed circuit board according to the priorart.

FIG. 3 is a sectional view showing the printed circuit board accordingto an exemplary, non-limiting embodiment of the present invention.

FIGS. 4A-4D are process sectional views showing a process of themanufacturing method of the printed circuit board according to anexemplary, non-limiting embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY, NON-LIMITING EMBODIMENTS

Various exemplary, non-limiting embodiments of the present inventionwill be described with reference to the accompanying drawings. It is tobe noted that the same or similar reference numerals are applied to thesame or similar parts and elements throughout the drawings, and thedescription of the same or similar parts and elements will be omitted orsimplified. In the following descriptions, numerous details are setforth to provide a thorough understanding of the present invention.However, it will be obvious to those skilled in the art that the presentinvention may be practiced without such specific details.

(A Printed Circuit Board)

A printed circuit board according to an exemplary, non-limitingembodiment of the present invention is explained using FIG. 3.

A flexible insulated substrate 1 having a first surface 1 a at one sideand a second surface 1 b at another side is made of a polyimide resin ora liquid crystal polymer. A wiring layer 2 a of a circuit pattern formedon the first surface 1 a is made of copper foil. The wiring layer 2 ahas parts corresponding to the reinforcement portions X1 and X2, such asa connector part or an element mounting part which needs reinforcement,and the deform able portion Y which does not require reinforcement.

The reinforcement plates 3 a and 3 b are positioned on the secondsurface 1 b at positions corresponding to the reinforcement portions X1and X2, respectively. The reinforcement plates 3 a and 3 b are placed onthe second surface 1 b with the auxiliary layers 4 a and 4 btherebetween. The plates 3 a and 3 b are secured to the surface 1 b bythermo-compression bonding.

A thin copper material with a thickness of about 10 micrometers, as theauxiliary layers, is formed on a surface of a thick metal material witha thickness of about 90 micrometers, as the reinforcement plates. Apattern mask is exposed on the surface corresponding to thereinforcement portions X1 and X2 by photolithography. Then, thereinforcement plates 3 a and 3 b, of stainless metal material, and theauxiliary layers 4 a and 4 b, of copper, are shaped by chemical etching.For example, the reinforcement plates and the auxiliary layers areformed in the shape of a rectangle.

A reinforcement edge side 3 as (3 bs) of the reinforcement plate 3 a (3b) is located at the outside of an auxiliary edge side 4 as (4 bs) ofthe auxiliary layer 4 a (4 b). In other words, the insulated substrate1, the undersurface 1 b of the insulated substrate 1, the auxiliary edgeside 4 as (4 bs) and the reinforcement edge side 3 as (3 bs) comprise aconcave wall. The auxiliary edge side 4 as (4 bs) is located in anindented position from the reinforcement edge side 3 as (3 bs).

Because of the position between the auxiliary edge side 4 as (4 bs) andthe reinforcement edge side 3 as (3 bs), the insulated substrate 1 isbent first at the auxiliary edge side 4 as (4 bs). Next, the insulatedsubstrate 1 is bent at the reinforcement edge side 3 as (3 bs) when thedeformable portion Y of the insulated substrate 1 is bent in a directionwhich brings the reinforcement plates 3 a and 3 b mutually closer.Therefore, a bending stress to the insulated board 1 is dispersed to theedge sides, and a concentration of the stress at one point is avoided sothat damage and fractures are mitigated.

Even when the auxiliary edge side 4 as (4 bs) and the reinforcement edgeside 3 as (3 bs) are in the same plane, the sharp edge of thereinforcement plate in the prior art 2 is not formed and damage andfractures are also mitigated.

As shown in FIG. 3, when each angle of inclination corner θ₁ between theauxiliary edge side 4 as (4 bs) and the second surface 1 b, andinclination corner θ₂ between the reinforcement edge side 3 as (3 bs)and the surface of the reinforcement plate 3 a (3 b) is changed to about90-degrees, the inclination is so gentle compared with the sharp edge ofthe reinforcement plate, in the prior art 2, that damage and fracturesare further mitigated.

Furthermore, as illustrated in FIG. 3, a boundary part 3 ap (3 bp) ofthe reinforcement edge side 3 as (3 bs) near the auxiliary layer 4 a (4b) is formed gentle curvature. Then the concentration of stress to theinsulated board 1 is avoided and also damage and fractures of theinsulated board 1 are mitigated.

(A Manufacturing Method of a Printed Circuit Board)

A manufacturing method of a printed circuit board with a reinforcementplate according to the exemplary, non-limiting embodiment of the presentinvention is explained using FIG. 3 and FIG. 4

First, as shown in FIG. 4A, for example, stainless steel, copper, copperalloy, brass, aluminum, phosphorus bronze, beryllium copper, or nickelis used as a reinforcement material 3. A copper plating layer with athickness of about 10 micrometers as an auxiliary material 4, is formedon the entire surface of a stainless steel material with a thickness ofabout 90 micrometers, as the reinforcement material 3. The etching speedof the auxiliary material 4 is faster than the reinforcement material 3.

Then, a thermoplastic resin film having a thickness from about 10 to 25micrometers, made of polyimide resin or a liquid crystal polymer, islaid on the entire upper surface of the auxiliary material 4, as theinsulated substrate 1. The copper foil thickness in a range from about 3to 35 micrometers is laid on the entire upper surface of the insulatedsubstrate 1, as an electrical conductive material 2.

Next, the auxiliary material 4, the insulated substrate 1 and theelectrical conductive material 2 are adhered directly bythermo-compression bonding, that is, by carrying out a press heatingoperation of the reinforcement material 3, the auxiliary material 4, theinsulated substrate 1, and the electrical conductive material 2 at atemperature in a range from about 260 to 300° C. In this process, byapplying a pressing force while heating at a temperature higher than theglass transition temperature Tg, mobility of the thermoplastic resin,which is the material of the insulated substrate 1, is increased. In thepress heating process, the thermoplastic resin adheres firmly to theuneven surface of the auxiliary material 4.

At least, the surface of the insulated substrate 1 should just consistof the thermoplastic resin. Therefore, to make the insulated substrate 1about 25 micrometers thick, a thermoplastic polyimide of about 2.5micrometers thickness is laid over both sides of the thermosettingpolyimide film such as a capton or an apical with a thickness of about20 micrometers.

Next, as shown in FIG. 4B, a pattern mask layer 5 of a photoresist isformed on the upper surface of the electrical conductive material 2 byphotolithography technology. The wiring layer 2 a with patternedcircuits is formed by removing the portion of the conductive materialexposed from the pattern mask layer 5, alternatively among theelectrical conductive material 2 (copper foil), by chemical etching asshown in FIG. 4C. Then, the pattern mask layer 5 is removed. Althoughnot illustrated, an insulating protection film may cover the wiringlayer 2 a surface, if needed.

As shown in FIG. 4D, the pattern mask layers 6 a and 6 b of thephotoresist are formed on the undersurface of the reinforcement material3 in the position of the wiring layer 2 a, corresponding to thereinforcement portions X1 and X2, which need reinforcement like theconnector part and the elements mounting part, respectively.

Then, chemical etching is carried out on the reinforcement material 3and the auxiliary material 4 until the second surface (undersurface) 1 bof the insulated substrate 1 is exposed. The etching is performed bysoaking the portion uncovered with the pattern mask layers 6 a and 6 bof the reinforcement material 3 in an etching liquid of ferric chloridesolution, as shows to FIG. 3. The etching liquid is about 38 to 50 Baumedegrees (specific gravity; about 1.35 to 1.53).

In the etching process of the reinforcement material 3 and the auxiliarymaterial 4, the portion of the reinforcement material 3 and theauxiliary material 4 corresponding to the deformable portion Y, isremoved alternately. Thus, the printed circuit board which has thereinforcement plates 3 a and 3 b and the auxiliary layers 4 a and 4 bshown in FIG. 3 is formed.

That is, the reinforcement plates 3 a and 3 b and the auxiliary layers 4a and 4 b are foamed substantially in the shape of a rectangle, byetching, using the pattern mask provided by photolithography technology,with a positioning accuracy of about ±20 micrometers or less,corresponding to the reinforcement portions X1 and X2, respectively.

Since the auxiliary material 4 has an etching speed faster than thereinforcement material 3 in the etching liquid, etching of the auxiliarylayers 4 a and 4 b tends to advance uniformly. The angle of inclinationof the corner θ₁ between the second surface 1 b and the auxiliary edgesides 4 as and 4 bs, is formed 90 degrees (i.e., is a quite obtuse anglecompared with the edges 36 a and 36 b shown in FIG. 2C as the prior at2). Therefore, a sharp edge part like the prior art be adjusted in 2 isnot formed in the auxiliary layers 4 a and 4 b close to the secondsurface (undersurface) 1 b of the insulated substrate 1. In addition,the angle of inclination of the corner θ₁ can be adjusted in the rangeof 70 to 90 degrees by etching control of the etching liquid, or theetching time, and so on.

Since the reinforcement material 3 is about 9 times thicker than theauxiliary material 4, the reinforcement edge sides 3 as and 3 bs of thereinforcement boards 3 a and 3 b have a large etching depth and a taperetching side indicated as the angle of inclination of the corner θ₂compared to the second surface 1 b (or upper surface of thereinforcement plate). However, it is easy to carry out etching controlof the angle of inclination of the corner θ₂ in the range of about 60 to80 degrees. The angle of inclination of the corner θ₂ is more gentlethan the sharp edge of the reinforcement plate of the prior art 2.

The boundary parts 3 ap and 3 bp of the reinforcement edge sides 3 asand 3 bs, near the auxiliary layers 4 a and 4 b (skirt of the taper) areprojections. But, the boundary parts 3 ap and 3 bp are separated fromthe second surface 1 b of the insulated substrate 1 by the auxiliarylayer 4. An abutting location of the projecting boundary parts 3 ap and3 bp to the lo insulated substrate 1, when the insulated substrate 1 isbent, mitigates the stress concentration at abutting portions. Moreover,since the projecting boundary parts 3 ap and 3 bp are formed with agentle curvature by etching, concentration of the abutting stress to theinsulated substrate 1 is avoided.

According to the positioning, the reinforcement edge side 3 as (3 bs) islocated at the outside of the auxiliary edge side 4 as (4 bs). In otherwords, the insulated substrate 1, the second surface (undersurface) 1 bof the insulated substrate 1, the auxiliary edge side 4 as (4 bs) andthe reinforcement edge side 3 as (3 bs) form a concave wall. Theauxiliary edge side 4 as (4 bs) is located in an indented position withrespect to the reinforcement edge side 3 as (3 bs).

Because of the position between the reinforcement edge side 3 as (3 bs)and the auxiliary edge side 4 as (4 bs), the insulated substrate 1 isbent first at the auxiliary edge side 4 as (4 bs), and next, at thereinforcement edge side 3 as (3 bs) when the deformable portion Y of theinsulated substrate 1 is subjected to substantial bending. Therefore,the bending stress to the insulated board 1 is dispersed to these edgesides, and the concentration of stress at one point is avoided so thatdamage and fractures are mitigated.

As mentioned above, the aspects of the exemplary, non-limitingembodiment are (1) the angle of inclination of the corner θ₁ between thesecond surface 1 b and the auxiliary edge side 4 as (4 bs) is formed tobe substantially 90 degrees; (2) the angle of inclination of the cornerθ₂ between the reinforcement edge side 3 as (3 bs) and the upper surfaceof the reinforcement plate (or the second surface 1 b) is more gentlycurved than the sharp edge of the reinforcement plate of the prior art2; and, (3) the reinforcement edge side 3 as (3 bs) Is located at theoutside of an auxiliary edge side 4 as (4 bs). Each of these aspects(1), (2), and (3) has the effect of preventing damage in bending orcurving of the insulated substrate 1, and when all of these aspectscooperate, damage and fractures are mitigated further.

According to the aspect (3), even if the auxiliary edge side 4 as (4 bs)and the reinforcement edge side 3 as (3 bs) are at the same face, thesharp edge of the reinforcement plate in the prior art 2 is not formedand damage and fractures are mitigated.

According to the manufacturing method of the exemplary, non-limitingembodiment, it is not necessary to use an adhesive layer that isdescribed in the prior art 1. Thus, adhesion of an unnecessary substancecan be avoided when the laminated structure shown in FIG. 4A is formed.

Moreover, the order of the patterning process of the wiring layer 2 aand the patterning process of the reinforcement plates 3 a and 3 b canbe suitably set. When the patterning process of the wiring layer 2 a isperformed where the auxiliary material 4 and the reinforcement material3 are laid on the undersurface of the insulated substrate 1, as shown inFIG. 4B and FIG. 4C, generation of creases or folds can be prevented inthe insulated substrate 1 and the electrical conductive material layer2. Therefore, the patterning accuracy of the wiring layer 2 a can bekept high, and it is especially effective in the manufacture of thinprinted circuit boards.

In addition, there are other methods of forming the laminating structureshown in FIG. 4A. Those are (A) a method of thermo-compression bondingof a sheet-like electrically conductive material for the electricalconductive material layer 2 on the spread insulation board 1 afterspreading the insulated board 1 on the auxiliary material 4 by applyingthe liquefied thermoplastic resin for the insulated boards 1 on thesurface of the auxiliary material 4 and drying; (B) a method of platingor sputtering the electrical conductive material for layer 2 on thespread insulation substrate 1 instead of the thermo-compression bondingof the sheet-like electrically conductive material in the method of the(A); (C) a method of adhering the reinforcement material 3 on theopposite side (the second surface) of the insulated substrate 1 throughthe auxiliary material 4 while a laminating object is preparedbeforehand to adhere the electrical conductive material layer 2 on oneside (the fit surface) of the insulated substrate 1. Each method ofthese (A), (B) and (C) does not require the adhesive layer, and it canbe applied alternatively according to the requirement, of the printedcircuit board, such as a adaptability, parts cost, or manufacturingcost.

Various combinations of the materials are possible if the etching speedof the auxiliary material is faster than that of the reinforcementmaterial. In this case, etching formation of the reinforcement plate orthe auxiliary layer can be ensured without etching the insulatedsubstrate 1 by using metal materials for each material. And the etchingcontrol is easy. Aluminum may be used as the reinforcement material 3instead of stainless steel. Iron or on iron alloy may be used as tieauxiliary material 4 instead of copper.

Although the auxiliary layer, which is thinner than the reinforcementplate, connects the reinforcement plate and the insulated substrate, atwo layer structure comprising the auxiliary layer and the reinforcementplate can be used as a reinforcement plate.

While the invention has been particularly shown and described withreference to the exemplary non-limiting embodiments thereof, the presentinvention is not limited to the embodiments. Note that without departingfrom the scope of the invention, it is possible for a person havingordinary skill in the art to make additions, deletions, replacements andother alterations. The scope of the invention is not limited by thedetailed description of specific embodiments, but is only defined by theclaims appended hereto, and their equivalents.

1. A printed circuit board, comprising: a flexible insulated substratehaving a first surface and a second surface; a wiring layer disposed onthe first surface; a reinforcement plate disposed on a portion of thesecond surface; and an auxiliary layer disposed between the secondsurface and the reinforcement plate, wherein, all auxiliary edge side ofthe auxiliary layer is indented from a reinforcement edge side of thereinforcement plate.
 2. The printed circuit board according to claim 1,wherein the reinforcement plate comprises a metal material.
 3. Theprinted circuit board according to claim 2, wherein the reinforcementedge side of the reinforcement plate is curved.
 4. The printed circuitboard according to claim 1, wherein the auxiliary layer comprises acopper material.
 5. The printed circuit board according to claim 1,wherein the reinforcement layer is thicker than the auxiliary layer. 6.The printed circuit board according to claim 1, wherein the angle ofinclination between the second surface and the auxiliary edge side issubstantially 90°.
 7. A method of manufacturing a printed circuit board,comprising: forming a wiring layer, by pattering, on a first surface ofa flexible insulated substrate; placing a reinforcement material on asecond surface of the flexible insulated substrate via an auxiliarymaterial; forming a pattern mask layer to partially expose a surface ofthe reinforcement material; and shaping an auxiliary layer and areinforcement plate by etching the exposed surface.
 8. The method ofmanufacturing a printed circuit board according to claim 7, wherein, theetching speed of the auxiliary material is faster than the reinforcementmaterial.
 9. A method of manufacturing a printed circuit board accordingto claim 7, wherein the auxiliary material is directly adhered to theflexible insulated substrate.
 10. A method of manufacturing a printedcircuit board according to claim 7 wherein the flexible insulatedsubstrate and reinforcement material are bonded each other.
 11. A methodof manufacturing a printed circuit board according to claim 7 whereinthe flexible insulated substrate and reinforcement material are bondedeach other.
 12. A method of manufacturing a printed circuit boardaccording to claim 9 wherein the auxiliary material is adhered to theflexible insulated substrate by thermo-compression bonding.
 13. A methodof manufacturing a printed circuit board according to claim 10 whereinthe flexible insulated substrate and the reinforcement material arebonded together by thermo-compression bonding.
 14. A method ofmanufacturing a printed circuit board according to claim 11 wherein theflexible insulated substrate and the reinforcement material are bondedtogether by thermo-compression bonding.